In the manufacture of metal sheets, such as steel sheets, it is common for these to be provided with a temporary corrosion protection for storage and transport immediately after the manufacturing or coating process and before delivery to the metal-processing industry. This is usually effected by applying a corrosion-inhibiting lubricant directly before the coiling of the sheet, such as a corrosion protection oil, a pre-lube or a dry-lube. If the metal sheet is destined for further processing by forming such as deep-drawing, the lubricants used can contain, in addition to corrosion-inhibiting substances, other ancillary substances, especially matched to the individual forming process, which improve the tribological properties of the sheet during the forming process. Metal sheets coated in this way are encountered, for example, in the automobile industry in the manufacture of bodywork made of metal sheets by deep-drawing.
With the use of lubricants for temporary corrosion protection and for improving the forming properties, it must be particularly borne in mind that these are to be easily and completely removable after the forming process.
From EP 0 489 105 B1 a method is known for the cold deep-drawing of metal sheets, in which the sheet which is to be deep-drawn and/or the deep-drawing tool are treated with an aqueous solution of an inorganic alkali phosphate and are then deep-drawn in the presence of a deep-drawing oil. The salt which is deposited on the sheet forms a soap under the pressure and temperature conditions of the deep-drawing process as a result of the reaction with the deep-drawing oil, which has the effect of reducing the friction coefficient. A disadvantage of the method is that the alkali phosphate used and the soap which is formed cannot be entirely removed from the sheet, free of any residue, after the deep-drawing.
From CH 441 594 a lubricant composition is further known for the cold-forming of metals without the removal of material, which contains a finely dispersed water-soluble inorganic salt for the improvement of the tribological properties, such as Borax. A disadvantage with this lubricant composition is that its lubrication capacity is not constant, since the salt is not distributed uniformly over the surface, which interferes with the deep-drawing process.
With the production of single-sided electrolytically galvanized fine sheet, very thin phosphating or borax passivation with a coating weight of approx. 10 mg/m2 has proved advantageous in respect of tribological properties. Both products can be applied in the existing flushing baths of the galvanizing lines, but they have the disadvantage that they may impair the phosphating process in the automobile production plant.
The use of prelubes takes account of the idea of influencing the tribological properties of steel strip directly on the surface of the steel. Prelubes contain drawing aids which are introduced into the oil coating, which is about 1 μm thick. Only the direct steel-tool interface is tribologically active, in the range of a few nanometers.
Finally, the principle is generally known of carrying out a surface treatment of the deep-drawn sheet in order to improve the tribological properties. In particular, chemical processes for the surface treatment are known, such as phosphating, by means of which the friction coefficient between the sheet and the tool is reduced, and the forming of the sheet section is thereby facilitated. Such a treatment, however, is expensive and cannot be applied to all deep-drawn sheets.